The objective of this research proposal is to evaluate cobalamin analogues for their ability to inhibit mammalian cobalamin-dependent enzyme reactions and interfere with the growth and metabolism of normal and malignant cells. A variety of cobalamin analogues will be synthesized containing substitutions in the upper axial ligand position, and substitutions and deletions in a number of other positions on the cobalamin molecule. These analogues will be evaluated in terms of their ability to restore enzymatic activity to mammalian apo-methylmalonyl-CoA mutase and apo-methionine synthetase, and to inhibit the restoration of activity that can be obtained with native cobalamin. We will also determine the ability of cobalamin analogues to enter cells, to be converted to their coenzyme forms, and to become bound to methylmalonyl-CoA mutase and methionine synthetase. The ability of cobalamin analogues to alter the growth and metabolism of normal and malignant cells in tissue culture and in vivo will be studied. An attempt will be made to obtain an animal model for the megaloblastic anemia and neurologic defects seen in human cobalamin deficiency and define the biochemical abnormalities responsible for these lesions. The effectiveness of cobalamin analogues as chemotherapeutic agents against a variety of mammalian malignancies will be evaluated.